5. THE
M
- MB, TOTAL CORRELATION:
BHs DO NOT KNOW ABOUT DISKS

It is important to note that BH mass does not correlate with disks in the
same way that it does with bulges.
Figure 4 shows the correlations of BH mass
with (left) bulge and (right) total luminosity.
Figure 4 (right) shows that
disk galaxies with small bulge-to-total luminosity ratios destroy the
reasonably good correlation seen in
Figure 4 (left). In addition,
Figure 4 shows
four galaxies that have strong BH mass limits but no bulges. They further
emphasize the conclusion that disks do not contain BHs with nearly the
same mass
fraction as do bulges. In particular, in the bulgeless galaxy M33,
the upper limit on a BH mass from STIS spectroscopy is
M 1000
M. If
M33 contained a BH with the
median mass fraction observed for bulges, then we would expect that
M
~ 3 × 107M.

Figure 4 tells us that BH masses do not "know
about" galaxy disks. Rather, they correlate with the high-density
bulge-like component in galaxies.

These results do not preclude BHs in pure disk galaxies as long as they
are small.
Filippenko & Ho
(2001)
emphasize that some pure disks are Seyfert
galaxies. They probably contain BHs. An extreme example is NGC 4395, the
lowest-luminosity Seyfert known (Fig. 4).
However, if its BH were radiating at
the Eddington rate, then its mass would be only
M ~ 100
M
(Filippenko & Ho
2001).
So disks can contain BHs, but their masses are much smaller in
relation to their disk luminosities than are bulge BHs in
relation to bulge luminosities. It is possible that the small BHs in
disks are similar to the seed BHs that once must have existed even in
protobulges before they grew monstrous during the AGN era.